Gents,
I have visited at least 20 of those former training bases in Québec , New Brunswick, PEI and Nova Scotia.
They all have the cement wall.As a matter of fact that cement wall is still in use in Saint-Jean Québec : A pistol range .
On advanced fighter training bases the wall probably would of been used to zero the machine guns or collimate them as per official vocab. At the time the most avanced fighter the Hurricane had .303 MG on both sides.
In Bagotville Québec ,home of 425 Hornet Squadron, the wall existed until very recently. I remember using it as a pistol and .22 rifle range.
I am almost sure it was used during the war to collimate the Mosquito Mg's there.
My two cents.
Cheerio !